This invention pertains generally to nickel-base superalloys castable as single crystal articles of manufacture, which articles are especially useful as hot-section components of aircraft gas turbine engines, particularly rotating blades.
The efficiency of gas turbine engines depends significantly on the operating temperature of the various engine components with increased operating temperatures resulting in increased efficiencies. The search for increased efficiencies has led to the development of heat-resistant nickel-base superalloys which can withstand increasingly high temperatures yet maintain their basic material properties. The requirement for increased operating temperatures has also led to the development of highly complex cast hollow shapes, e.g., blades and vanes, which provide efficient cooling of the material used to produce such shapes.
The casting processes used with early generations of nickel-base superalloys, commonly referred to as conventionally cast nickel-base superalloys, generally produced parts whose microstructures consisted of a multitude of equiaxed single crystals (grains) of random (nonoriented) crystallographic orientation with grain boundaries between the grains. Grain boundaries are regions of highly nonoriented structure only a few atomic diameters wide which serve to accommodate the crystallographic orientation difference or mismatch between adjacent grains.
A high angle grain boundary (HAB) is generally regarded as a boundary between adjacent grains whose crystallographic orientation differs by more than about 5-6 degrees. High angle grain boundaries are regions of high surface energy, i.e., on the order of several hundreds of ergs/cm.sup.2, and of such high random misfit that the structure cannot easily be described or modelled. Due to their high energies and randomness, high angle grain boundaries are highly mobile and are preferential sites for such solid-state reactions as diffusion, precipitation and phase transformations; thus, high angle boundaries play an important role in the deformation and fracture characteristics and chemical characteristics (e.g., resistance to oxidation and hot corrosion) of polycrystalline metals.
Also, due to the high energies and disorder of HABs, impurity atoms are attracted preferentially (segregated) to high angle grain boundaries to the degree that the concentration of impurity atoms at the grain boundary can be several orders of magnitude greater than the concentration of the same impurity atoms within the grains. The presence of such high concentrations of impurity atoms at high angle grain boundaries can further modify the mechanical and chemical properties of metals. For example, in nickel-base superalloys, lead and bismuth are deleterious impurities which segregate to the grain boundaries. At high temperatures, even small amounts (i.e., a few ppm) of such impurities in the grain boundaries of nickel-base superalloys degrade the mechanical properties (e.g., stress-rupture strength) and failure generally occurs at the grain boundaries.
In contrast to high angle grain boundaries, low angle grain boundaries, sometimes also called subgrain boundaries, are generally regarded as boundaries between adjacent grains whose crystallographic orientation differs by less than about 5 degrees. It is to be understood, however, that the classification of a boundary as high angle or low angle may vary depending upon the person or organization doing the classification. For the limiting case of a low angle boundary (LAB) where the orientation difference across the boundary may be less than 1 degree, the boundary may be described (modelled) in terms of a regular array of edge dislocations, i.e., a tilt boundary. While the mismatch is technically that between any two adjacent grains, and not that of the boundary per se, the extent of the mismatch is commonly assigned to the boundary; hence the terminology of, for example, a 5 degree low angle boundary, which usages shall be used herein interchangeably.
Low angle grain boundaries are more highly ordered and have lower surface energies than high angle grain boundaries. Higher order and lower energy result in boundaries with low mobility and low attraction for impurity atoms which, in turn, results in a lesser effect on properties, mechanical and chemical, compared to high angle grain boundaries. Thus, while no grain boundaries constitute a preferred condition, low angle boundaries are to be preferred over high angle grain boundaries.
Improvements in the ability of conventional superalloys to withstand higher temperatures without impairing other needed qualities, such as strength and oxidation resistance, was achieved through alloy development and the introduction of improved processing techniques. These improvements followed from findings that the strength of such superalloys, and other important characteristics, were dependent upon the strengths of the grain boundaries. To enhance such conventional superalloys, initial efforts were aimed at strengthening the grain boundaries by the addition of various grain boundary strengthening elements such as carbon (C), boron (B), zirconium (Zr), and hafnium (Hf) and by the removal of deleterious impurities such as lead (Pb) or bismuth (Bi) which tended to segregate at and weaken the grain boundaries.
Efforts to further increase strength levels in conventional nickel-base superalloys by preferentially orienting the grain boundaries parallel to the growth or solidification direction were subsequently initiated. Preferential orientation of the grains generally results in a columnar grain structure of long, slender (columnar) grains oriented in a single crystallographic direction and minimizes or eliminates grain boundaries transverse to the growth or solidification direction. The process used, i.e., directional solidification (DS), had long been used for other purposes such as the manufacture of magnets and grain-oriented silicon steel for transformers. That process has been described and improved upon, for instance, in U.S. Pat. No. 3,897,815--Smashey. The disclosures of all the U.S. Patents referred to herein are hereby incorporated by reference.
Compared with conventionally cast superalloy articles, directionally solidified (DS'd) articles exhibited increased strength when the columnar grains were aligned parallel to the principal stress axis due to the elimination or minimization of grain boundaries transverse to the direction of solidification. In addition, DS provided an increase in other properties, such as ductility and resistance to low cycle fatigue, due to the preferred grain orientation. However, reduced strength and ductility properties still existed in the transverse directions due to the presence of longitudinal columnar grain boundaries in such DS'd articles. Additions of Hf, C, B, and Zr were utilized to improve the transverse grain boundary strength of such alloys as was done previously in conventional equiaxed nickel-base superalloys. However, large additions of these elements acted as melting point depressants and resulted in limitations in heat treatment which did not allow the development of maximum strengths within such directionally solidified superalloys.
It has been recognized for some time that articles could be cast in various shapes as a perfect single crystal, thus eliminating grain boundaries altogether. A logical step then was to modify the DS process to enable solidification of superalloy articles as single crystals to eliminate longitudinally extending high angle grain boundaries previously found in DS'd articles.
In the single crystal metallic alloy arts, it has heretofore been conventional teaching that elements such as boron, zirconium, and carbon are to be avoided, i.e., kept to the lowest levels possible with commercial melting and alloying practice and technology. For example, U.S. Pat. No. 3,494,709 recites the deleterious effect of B and Zr, proposing limits of O.001% and O.01% for those elements, respectively. U.S. Pat. No. 3,567,526 teaches that the fatigue properties of single crystal superalloy articles can be improved by the complete removal of carbon.
In U.S. Pat. No. 4,116,723, there is disclosed homogeneous single crystal nickel-base superalloy articles having no intentional additions of cobalt (Co), B, Zr or C which are said to have superior mechanical properties, e.g., creep and time to rupture, compared to similar nickel-base superalloys containing Co, C, B, and Zr. Therein it is taught that cobalt should be restricted to less than about 0.5%, and more preferably to less than about 0.2%, to preclude the formation of deleterious topologically close packed phases (TCP) (e.g., .sigma. and .mu.). Furthers it is taught therein that no single element of the group carbon, boron, and zirconium should be present in an amount greater than 50 ppm, that preferably the total of such impurities be less than 100 ppm and, most preferably, that carbon be kept below 30 ppm and that B and Zr each be kept below 20 ppm. In any event, it is taught that carbon must be kept below that amount of carbon which will form MC type carbides. Subsequently, in U.S. Pat. No. 4,209,348 it was shown that 3-7 % Co could be included in the single crystal nickel-base superalloys disclosed there in without forming TCP.
Another purpose in limiting C, B, and Zr is to increase the incipient melting temperature in relation to the gamma prime solvus temperature thus permitting solutionizing heat treatments to be performed at temperatures where complete solutionizing of the gamma prime phase is possible in reasonable times without causing localized melting of solute-rich regions. Recently, however, it has been recognized, U.S. Pat. 4,402,772, that the addition of hafnium in small amounts to certain of nickel-base superalloys for the casting of single crystal articles is effective, or example, in providing enhanced properties and enhanced heat treatability in that such articles have a greater temperature range between the gamma prime solvus and incipient melting temperatures than do most prior art single crystal articles.